Mapping the directional elasticity in living human skin with air-coupled ultrasound and light

Ivan (Vanya) Pelivanov –

University of Washington (UW), Department of Bioengineering, 616 NE Northlake Pl, Benjamin Hall bld, room 363, SEATTLE, WA, 98105, United States

Mitchell A. Kirby, Peijun Tang, Gabriel Regnault, Maju Kuriakose, Matthew O’Donnell, Ruikang K. Wang
University of Washington, Department of Bioengineering

Russell Ettinger
University of Washington, Burn and Plastic Surgery Clinics at Harborview

Tam Pham
University of Washington, Regional Burn Center at Harborview

Popular version of 4aBAa2 – Quantification of Elastic Anisotropy of Human Skin in vivo with Dynamic Optical Coherence Elastography and Polarization-sensitive OCT
Presented at the 184 ASA Meeting
Read the abstract at

We believe that mapping skin elasticity with sub-mm resolution may have tremendous impact in dermatology, transplantology and plastic surgery, dramatically improving current monitoring of wound healing and tissue recovery, reducing surgical failure rates, providing immediate quantitative feedback on all procedures, and opening many new opportunities for reconstructive medicine.

Skin grafting is one of the oldest and most widely used reconstructive techniques, finding clinical applications across many surgical and cosmetic areas. Factors related to skin’s elastic properties (such as contractions and shearing forces) are among the most common complications of full thickness skin grafts (FTSGs). Recent studies show that the recipient site work best when its elastic properties are matched by transplanted donor tissue. With tens of millions of aesthetic procedures performed every year in the USA, surgical cosmetology is clearly critical, especially when procedures are performed on the face, neck or breast. Currently there are no tools that can quantitatively map skin’s elasticity in living people.

What does elasticity mean for soft tissue? In general, tissue elasticity defines how it changes shape due to an applied external force. It can be complicated depending on tissue structural organization. For many tissue types like kidney, liver, or breast), however, elastic properties are isotropic (that is, independent of the direction of applied force) and can be described by a single parameter called the shear modulus. This parameter has very important diagnostic power because it correlates well with what a physician feels when compressing also known as palpating, tissue. Hematoma, different lesions and nodules, cysts, or scar feel very different compared to normal tissue due to shear modulus changes.

What do we propose? Skin is a complex organ with directional dependence of mechanical properties mainly governed by the local orientation of collagen fibers in the dermis. This means that skin deforms differently when it is stretched in different directions, for instance, either along or across fibers. To characterize skin’s arbitrary deformation, a single shear modulus (as for isotropic organs) is not enough; instead, 3 independent elastic moduli are required. We propose to map these moduli in skin using a noncontact, fully non-invasive method, with sub-mm spatial resolution and nearly in real time. We hypothesize that quantifying skin elasticity in living patients will enable significant innovation within all areas of dermatology and plastic, burn, or oncologic surgery, that will modify a patient’s tissue quality and reduce unintended outcomes from medical, radiologic, or surgical intervention.

How do we measure elastic properties in skin? Over the last twenty-five years, elastography using magnetic resonance imaging (MRI) and ultrasound systems has evolved from an interesting concept into an important clinical tool. In skin, however, MRE resolution is insufficient, and no contact as in ultrasound, can be applied to tissue for many important medical conditions. Our method is based on noncontact dynamic Optical Coherence Elastography (OCE) where mechanical waves in skin are launched with an air-coupled acoustic transducer, meaning, through air, and recorded in space and time with Optical Coherence Tomography (OCT, Fig. 1a). Video snapshots clearly show high variation in the surface wave speed (Fig. 1c) for different, even close body sites (Fig. 1b). In addition, different OCT modalities can measure skin’s structure (Fig. 2e), local fiber orientation (Figs. 2c, g) and its vascularization (Fig. 2f), providing very rich information on its structural and functional properties.

Figure 1. (a) – Diagram of Optical Coherence Elastography (OCE) measurements in human skin. (b) – Example imaging sites in palm and wrist. (c) – Snapshots of propagating mechanical waves over skin surface in two imaging locations and corresponding wave speed maps at these locations. Click here to see the full video. Image courtesy of [SOURCE]

Our findings: We studied skin elasticity in healthy volunteers in vivo. By measuring the speed of mechanical waves propagating in different directions (Fig. 2a) along the skin surface in the forearm (Fig. 2b), we determined all three elastic moduli in skin and identified local collagen fiber orientation (blue dashed line in Fig. 2b). Polarization-sensitive Optical Coherence Tomography produced the same fiber orientation (red dashed line in Fig. 2b) from pure optical measurements (Fig. 2c). We also showed that all parameters differ markedly in scar (Fig. 2d) compared to surrounding normal skin (Figs. 2e-h).

Figure 2. (a) – Diagram of Optical Coherence Elastography (OCE) scanning orientation in the forearm in vivo. (b) – mechanical wave anisotropy in human skin with reconstructed collagen fiber orientation and elastic indexes. (c) – imaging fiber orientation with polarization-sensitive Optical Coherence Tomography (PS-OCT). Dashed blue and red lines in (b) correspond to the local fiber orientation reconstructed with OCE and PS-OCT respectively. (d) – imaging of human scar with structural OCT (e), OCT angiography (f), PS-OCT (g) and OCE (h). Images were adapted from Image courtesy of [SOURCE]